JP-2009-85164A (US-2009-0088951A1) shows a fuel injection system which is provided with a fuel pressure sensor detecting a fuel pressure in a fuel passage between a common-rail (accumulator) and an injection port of a fuel injector. Based on a detection value of the fuel pressure sensor, a fuel pressure waveform indicative of a variation in fuel pressure due to a fuel injection is detected. If the detected fuel pressure waveform is significantly different from a supposed waveform, it is determined that a fuel injector has a malfunction in which a valve body is stuck at its opening position so that the fuel is continuously injected through the fuel injector.
However, in a case that the above system is applied to a multi-cylinder engine, it is necessary that the fuel pressure sensor is provided to each of fuel injectors, which may increase its costs.